For those who don't need a background story to go with the question, I'll get straight to it: What is the maximum file size (in KB, not dimensions) that can be used for images in Mobi files? What about AZW3 files?

For those who are willing to read a background story -- who knows, it might provide some useful context -- here goes.

I owned a Sony reader for years, but am now converting my library to use on a Kindle Paperwhite. Some of my old ePubs have images in them that are ~250 KB. They always showed up on my Sony reader at full quality -- or at least what appeared to be full quality to my middle-aged eyes. However, so far my experiments in converting these ePubs to Mobi with Calibre have resulted in a noticeably lower level of image quality. Artifacts, blurriness, loss of detail -- not exactly what I like to see when referencing a historical map or painted portrait, even if it is in black and white.

I've done my best to research the issue, and have learned that images with "large" file sizes are compressed during conversion to Mobi. However, I cannot figure out what constitutes a "large" file size. What is the "magic number" of KB over which the images will get compressed, and under which they are left alone? I found a reference to 127 KB being the limit, but I also found a reference to 250 KB being the limit.

I would be most grateful to anyone who can teach me. My goal is to downgrade the "large"' images on my own, to maximize their quality while still appeasing the Kindle. Obviously, knowing my upper limit would be terribly helpful.